Hi Everyone, I'm new. I'm a native English speaker with some experience trying ONLY to hope students modify their accented (but grammatically correct for the most part) English. I've noticed that irregular/atyipical stress is one of the big contributors to perceived poor accent. Stress as in timing, phrasing and emphasis.The other really big one is distorted vowels.
I agree with the suggestion of having the students read kids books, such as Dr. Suess, if you and they will tolerate it. Also, what has worked well for me is making lists of contrastive pairs, of English words containing the sound they mispronounce, and another English word containing a closely related phoneme. For example: "bear" vs "bar". First you have to make sure the student really can distinguish between the two sounds. A Russian speaker of mine could not differentiate some of our vowel phonemes. Then have the student read the list slowly to you and be really strict about what you accept as correct. If they can get a few sounds down, and memorized, they can begin to generalize.
I also agree with the other poster, that if you as a teacher try to mispronunce the sound in the way your student does, you can figure out what he/she is doing wrong and show them the contrast as you "fix" it in your own production.
Hope this helps!